[Bug c/124830] c/c-decl.cc Fails to cross compile with C++20 host compiler

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124830

--- Comment #6 from Drea Pinski <pinskia at gcc dot gnu.org> ---
 basically INCLUDE_MEMORY, INCLUDE_MUTEX, and INCLUDE_SSTREAM all need to moved
to before the include of safe-ctype.h

GCC 15 INCLUDE_SSTREAM was handled with
r15-5661-gf6e00226a4ca63e76e3e0b3a09a4ce6223980981

and MEMORY was done with r15-5603-gb3f1b9e2aa079f (by making it uncondtional).

MUTEX is not done on the trunk though.